Silver-glass die attach adhesives are being used increasingly in the electronics industry to secure silicon based integrated circuit devices to their packages. These adhesives have both practical and economic advantages compared to the traditional Si/Au eutectic attachment methods. The Ag/glass adhesives typically consist of silver flake and glass particles suspended in an organic matrix or vehicle. The traditional vehicle system for a silver-glass paste composition has consisted of a resin binder dissolved in one or more solvents. See for example, U.S. Pat. No. 4,636,254, granted Jan. 13, 1987 and assigned to Quantum Materials, Inc. The subsequent processing (cure) of an Ag/glass paste results in the concurrent evolution of the organics, fusion of the glass, and sintering of the silver flakes.
One essential requirement for proper performance of an Ag/glass paste is the substantial elimination of vehicle organics during cure. These organic materials of necessity must be capable of total (and non-carbonizing) volatilization or decomposition during the subsequent firing of the product to develop its adhesive properties. It is furthermore desirable that this organic burnout be essentially complete prior to the glass transition temperatures (T.sub.g) of the frit. The lead borate glasses traditionally used in the Ag/glass compositions are easily reduced by residual organics at such temperature and their adhesive properties are correspondingly diminished. Therefore, preferred solvents for these adhesives have boiling points well below 300 degrees C. Preferred resins that are considered to have adequate burnout characteristics include polyalkyl methacrylates and nitrocellulose (preferable .gtoreq.12.0% nitrogen by analysis). The methacrylate resins are usually fully decomposed to volatile compounds between 300.degree. and 400.degree. C.
Resin binders have been used in the traditional silver-glass formulations by reason of the following properties they confer to the paste:
1) They help keep the paste solids in suspension.
2) Their presence helps retard solvent bleed from the paste after decomposition on a substrate.
3) They impart some "green strength" to the paste during any drying step.
There are certain disadvantages, however, that are associated with the use of these binders in a silver-glass die attach paste, as follows:
1) Dissolved resin impedes the release of solvent vapors from the paste solids during processing.
2) The presence of more than a fraction of one percent of these resins can reduce the thixotropic index of a paste and thus have a negative effect on dispensability.
3) New, lower temperature processing compositions require that all organic residues be fully volatilized by 300.degree. C.
Use of these resin polymers can also compromise product performance in other ways. The thermal coefficient of expansion for polymethyl methacrylate, for example, is more than an order of magnitude greater than that of the silicon die. It is believed that this thermal mismatch contributes to bond failure, particularly on large area devices. Nitrocellulose is an even less desirable resin alternative since its burnout is too rapid (resulting in partial delamination) and incomplete (giving rise to excessive carbon residue).